The invention relates to a discharge lamp and to a headlight for a motor vehicle.
Besides the incandescent lamps which have been used for a long time in automobile application, discharge lamps are also increasingly used for motor vehicle lighting. A gas discharge which emits a very intensive light is generated in a closed discharge vessel between two electrodes in known discharge lamps.
A discharge lamp usually comprises a lamp base, a burner comprising a discharge vessel, and an outer bulb. The lamp base serves to retain the burner and to position the lamp and contact it electrically in a headlight. The actual light generation takes place in the discharge vessel. Usually, an outer bulb of glass is arranged around the discharge vessel. This bulb serves to filter out UV radiation from the emitted light.
A motor vehicle headlight comprises a reflector and a lamp projecting through an opening into the interior of the reflector. The lamp is exactly positioned with its lamp base in a holder of the reflector, so that the discharge vessel is arranged in an exactly defined location. The reflector surface is illuminated by the emitted light. A headlight beam is formed thereby which, depending on the shape of the reflector surface, is radiated in forward direction. Various specifications exist for such headlight beams, depending on the lighting function to be performed. Thus a low beam, for example, must exhibit an asymmetrical light distribution with a substantially horizontally sharply delimited bright/dark cut-off, so that oncoming traffic is not dazzled. In a high beam, on the other hand, the maximum light intensity should be concentrated on the optical axis, while the remaining light distribution may be approximately symmetrical, and there is no bright/dark cut-off.
Although the discharge lamp has a number of advantages over conventional halogen lamps, it is considerably more difficult in a discharge lamp, as compared, for example, with known H4 lamps, for one lamp or one headlight with one lamp to be able to fulfill two lighting functions at the same time.
Thus DE-A-4435507 describes a headlight with a reflector and a light source which is arranged in a lamp holder, which lamp holder can be pivoted relative to the reflector by means of an adjustment element about a horizontal axis between a position for the low beam and a position for the high beam. In addition, movable screens are shown, which screens move during the pivoting movement from the low-beam to the high-beam position such that a lower region of the reflector, which is cut off in the low-beam position, is illuminated in the high-beam position.
DE-A-19825375 describes a headlight with a gas discharge lamp for motor vehicles, which headlight can be screened and unscreened. The gas discharge lamp is incorporated in a fixed position in the reflector housing of the reflector. A polarizing filter, which can be made transparent and opaque so as to serve as the control adjustment member, is arranged in the reflector housing such that a switch-over can be made between the low-beam and high-beam functions.
EP-A-0324652 describes an automobile headlight with a parabolic reflector and a gas discharge lamp. The gas discharge lamp is positioned in the focus of the reflector. To switch over between high beam and low beam, the light arc of the lamp is deflected through the generation of an electric or magnetic field such that a high beam is generated in conjunction with the reflector and the light arc in a first position, and a low beam in another position.
GB-A-2216332 describes a headlight for an automobile in which a gas discharge lamp is arranged inside a reflector. The gas discharge lamp generates a very concentrated light at its cathode. The location of the light generation is switched over between the electrodes arranged at a distance from one another through a change in the polarity, so that a low beam or a high beam is generated owing to different reflections on the reflector.
The existing solutions have a few practical disadvantages. In the case of a mechanical movement of the burner, it is very complicated to achieve the necessary exact positioning inside the reflector. The other solutions also proved to be complicated and not satisfactory in all cases.
Against this background, it is an object of the invention to provide a lamp and a headlight for a motor vehicle in which a switch-over between two lighting functions takes place in a simple manner, while nevertheless an exact positioning of the discharge vessel is maintained.
This object is achieved by means of a discharge lamp as claimed in claim 1 and a headlight as claimed in claim 6. Dependent claims relate to advantageous embodiments of the invention.
According to the invention, a moving member comprising at least one optically active element is provided at the lamp. The moving member is movably provided at the lamp, so that it can assume at least two different positions with respect to the discharge vessel.
The moving member comprises at least one optically active element. Preferred elements which may be used are lenses, but, for example, also prisms or mirrors, in particular concave mirrors. The optically active element generates an image of the gas discharge in the discharge vessel which is shifted with respect to the actual location of the gas discharge. The result of this is that the gas discharge seems to be shifted when observed from a location optically situated behind the optically active element.
The moving member is movable between at least two positions, so that it can be actively controlled whether from a given direction the discharge vessel with the discharge taking place therein is directly visible, or the shifted image thereof is visible through the optically active element.
A headlight according to the invention comprises a reflector and a lamp with a moving member. A certain region of the reflector is illuminated, in dependence on the position of the moving member, either directly by the gas discharge or indirectly through the optically active element, i.e. by the shifted image of the discharge generated by the element. The discharge vessel appears to be in its actual position or in a shifted position viewed from this region of the reflector, in dependence on the position of the moving member.
A movement of the actual light-generating element, i.e. the discharge vessel, is not necessary with the lamp according to the invention and the headlight according to the invention. The discharge vessel and the entire burner can indeed remain in place, which is advantageous in particular in respect of their fixed retention, exact positioning, and electrical connection. The moving member may be directly attached to the lamp, for example to the lamp base. It is alternatively possible, however, to arrange the moving member separately in the reflector.
The invention achieves the effect that the light-generating discharge vessel is seemingly switched between two positions, depending on the position of the moving member, for at least certain reflector regions. As is known from the cited prior art and also from conventional H4 lamps, two lighting functions can be realized with one reflector through shifting of the light-generating element. It is known in detail to those skilled in the art how a displacement of the light source influences a light beam emitted by a reflector. This effect is utilized, for example, with dual-filament incandescent lamps (for example H4), in which two different light beams are generated with one reflector and two coils arranged at a small distance from one another, such that two lighting functions are performed. With a parabolic reflector, for example, a slight displacement from the focus in forward and upward direction leads to a distribution which causes a low-beam distribution shifted in downward direction with respect to the parallel light beam. The shifting of the light source in conjunction with specially designed xe2x80x9cindependent surfacexe2x80x9d or xe2x80x9ccomplex shapexe2x80x9d reflector surfaces, whose shapes are digitally determined for the relevant application, renders it possible to generate two different light beams whose requirements are each defined beforehand and are taken into account in the design of the surfaces.
In a further embodiment of the invention, it is proposed that the moving member is a sleeve of a transparent material which surrounds the discharge vessel. In a particularly advantageous embodiment, this sleeve is the outer bulb of the discharge vessel. Said outer bulb, which has served mainly as a UV filter until now, is fixedly connected to the discharge vessel in conventional applications, usually by fusion. This, however, is not necessarily the case. This is why the outer bulb may be used as the moving member.
The outer bulb is usually arranged longitudinally with respect to the optical axis. It may be axially shifted when serving as the moving member. It is preferred, however, that the moving member is capable of rotation, in particular about the optical axis. A satisfactory support and an exact positioning are easy to achieve in that case. The operation may be realized in various ways. The use of mechanical adjustment drives as well as a direct drive by an electric motor is possible. Piezoactuators and the use of electromagnets are also conceivable.
In an advantageous embodiment, the optically active element is provided at the outer bulb. Preferably, there are two optical elements which are provided at mutually opposed sides of the outer bulb and which are active in a plane perpendicular to the optical axis on which the light source is situated, each over an angular range of 25 to 120xc2x0, preferably approximately 60 to 90xc2x0.
Lenses are preferred as the optically active elements, which lenses are formed such that they cause the gas discharge to appear to be displaced, preferably in the direction of the optical axis, when viewed through the lens. Suitable lenses may be readily designed by those skilled in the art for the respective applications.